Tack driving device

ABSTRACT

A gun for driving tacks or other wide headed nails is disclosed wherein the head of each tack or nail to be driven is engaged by means supported by the percussion pin and mobile therwith along the drive channel, so that during all the nail driving stroke the tack or nail is kept perpendicular to the material to be riveted, these means comprising a pair of gripping members which are brought into engagement with the head of the nail to be driven and which towards the end of the stroke of the percussion pin and thus of the gripping members themselves, release the head of the nail being driven into the material to be riveted.

O United States Patent 11 1 7 1111 3,764,054 Monacelli [45] O t, 9, 1973 TACK DRIVING DEVICE I [76 Inventor: Umberto Monacelli, Via Solferino, Primary Examiner-Granville Custer 3], Udine, Italy Attorney-B. Edward Shlesinger et a1. [22] Filed: Dec. 20, 1971 [57] ABSTRACT [211 App]. No.: 209,646 i A gun for driving tacks or other w1de headed nails is disclosed wherein the head of each tack or nail to be [52] US. Cl. 227/114, 227/138 driven is engaged by means supported by the percus- [51] Int. Cl. B25c 1/04 sion pin and mobile therwith along the drive channel, [58] Field Of Search 227/114, 115, 116, so that during all the nail driving stroke the tack or 227/1 8, 1 9 nail is kept perpendicular to the material to be riveted, these means comprising a pair of gripping members I Reffil'ellces Cited which are brought into engagement with the head of UNITED STATES PATENTS the nail to be driven and which towards the end of the 2,878,954 3/1959 Troske 227/118 of the Percussion and thus gripping 1,483,151 3/1924 McCormick 227/114 members themselves, release the head of the nail 1,501,421 7/1924 Stimpson 227 114 being driven into the material to be riveted. 3,061,835 11/1962 Hain 227/147 3,521,398 7/1970 Coutts 227 149 6 Claims 15 Drawing Figures AVIAWIIIIIIIIIII I n n I I n l l Patented Oct. 9, 1973 6 Sheets-Sheet 1 CD N INVENTOR {3 Mon 4 c [(5 1M BY M ATTORNEY-7 Patented Oct. 9, 1973 3,764,054

6 Sheets-Sheet 5 36a 23a 23a INVENTOR l/mbufo mumm- PGI'TORN Patented Oct. 9, 1973 3,764,054

6 Sheets-Sheet 4 INVENTOR um barf?) N na Le. BY

Patented Oct. 9, 1973 3,764,054

' 6 Sheets-Sheet 5 .TACIKHDRIVING DEVICE The present invention relates to a, gun for driving tacks or other wide headed nails.

Compressed air-riveting guns are available in which staples are fed to a ,drive channel in which they are guided during their travel following a blow imparted by a hammer or percussion pinactuated by compressed air to drive them into the wood or other material to be riveted.

Thesecompressedair guns however have been found unsuitable for riveting wide headed nails, particularly the common tacks used by upholsterers. It often happens thattherheads of the tacks, asthey slide along the guide walls ofthe drive channel, become removed, deformed or inclined with ;the result that the tacks are madetoemerge withtheir shanksobliquewith respect to the required direction.

The .object of the present invention is consequently to'provide aigunfifor driving tacks or other wide headed nails which. does'not present the.aforementionedtdisadvantages andinstead allowsxthettacks or like nailsto be driven intothe materialzto. be rivetedtunder conditions of perfect structural;integrityand.perpendicular to the plane of insertion.

The aforementioned object iso'btainedby means of a gun =whichcomprises,gripping:meanssupported by .the percussion "pin and mobile with .it :along the .firing .channel, said means .duringat-he strokeofthe percussion pintowards th'eAfirin-gmouth being maintained in a closed positionisupporting the head. of :the tack .until a point close :to :the =..end of ithe--stroke of l the :percussion ;.pin, beyond which :said means are .automatically opened. in orderztol free. the "head of thetack: and .allow said tack freely to be inserted into the material :to be riveted .underthe further thrusttof therpercussiongpin. In this mannerithe head oftheitackx isadequatelysup- ;ported and guided (in aimanner which does notgrprejudice its structural: integrity :because: sliding against the walls of the. drive channel, whichmight damage it, is no longer-necessary) along "the -greater1partof itsrstroke towards the firingumouth andQisffreedonly at the last part of said stroke, .in particular when a :part of the shank has already'penetrated the materialto besriveted, i.e when the support and guide are no longer necessary and in factit is indispensablethat theitack-is left free in order to allow.complete insertion of the shank into the material to be riveted. Inrthis manner-separation,.deformation orinclination of the tack head no longer occursand said tack may bedriven int o the materialitoibe riveted under conditionsofperfect structural integrity andperfectly perpendicular to the :plane. of insertion.

The characteristicszand advantages of the present invention will 'be moreevidentafter an examination of the following detailed description of some embodiments of theggunaccordingtotheinvention. In the followingdetailed description, given by way of example, reference is made to the accompanying drawings in which:

FIG/1 is apart sectionaLapart lateral view of a first embodiment of agun according'to the invention;

F1652 is a sectional view .onfthe line II--II.o'f FIG. l with thepercussionxpin in the restpposition;

FIG. 32is a sectional view on theline IlI-,III of-FIG.

FIG. 4 is afrontal elevation of one of two joined plates which form the drive channel of the gun shown in the preceding Figures; 1

FIG. 5 is an elevation of the plate of FIG. 4 from within;

FIG. 6 is an elevation of the other plate from within;

FIG. 7 is a plan view from below of the assembly comprising the two plates;

FIG. 8 is a sectional view of the type shown in FIG. 2, but with the percussion pin advanced to the position in which the gripping members are openedfor discontinuing the supporting action for the head of the tack, whose shank is already partially driven into the material to be riveted;

FIGS 9 andll are sectional views'similar to thoseof FIGS. 2 and'8,but refer to a second embodiment ofthe gun according to the invention;

FIG. .1 l. isa sectional viewon the line XIXI.ofFIG.

FIGS. "12 and 13 are sectional views similarto those of'FIGS. 2, 8 and 9, but referto a thirdembodiment of thegun according to the invention;

*FIGS. 14 and 15 are sectional views which show a fourth embodimentof the gun according to the invention withthepercussion pin in the two extreme conditions, respectively namelythe restposition andposition of maximum advancementffor driving the tack into the material to be riveted.

The gun shown insFlGS. ll -18 comprises essentially a working head :1 operatedbycompressed air, a drive .chann'el 2,.a magazine Storloading thetacks'into the firing channel, .an air reservoir 4 constituting athand gripffor' feeding compressed air to the head .l,..a:nipple "Sforconnecting the arm Mrto a source of compressed airl(not shown),an operatingtriggerc6 and a ma gazine supportand gri 1-7 The worki 'lg head #1 :comprises.a li d g which is .fedat itsupper end with compressed air suppliedto it by the5feed arm 4 by way of a valve 9.andwhas inits lower end apad :10 which actsasya stoptfor.apiston L1 1 which slides -within' the .cylinderi8 and is; provided with .a cylindrical rod "112 which constitutes the ,;percussion pin, pusher or hammer of ithe gunhlnside.the percussion pin :12 a.c ylindricalrod 13 is disposed axially and provided with two .enlargedend heads 14 and 15, by' way of thefirst of which itis adjustably attached tolthe .pis- [011 111. A bushing ill6 isvslidablymounted.on the rod-l3 and is thrustbymeans of.aspringillflxwound about. the rod 1 3 and interposedbetween therbushing :1'6 andwen larged head i 14, "towards the enlarged head I105. The upper'en'dsiil9 of two gripping. memberscomprisingtwo jaws are inserted "between the bushing 16 and .enlarged head 15, passing through respective .diametrically-opposed apertures 18 in .thepercussion pinflZJAs can beseenfrom FIGS. 2 and :8, these upper :ends 19 are bent inwardlysso as .to-form an angle ofabout with respectxtoithe imid'dlegportions of the, jaws. The lower endsl l .arelbent at right angles inwardly with respectltorthe same rn'icldle gportions.

Both the lower end. portion oflthe pin l 2.and..almost the whole length or the;1 jawsrzfll are constantly. inserted in theldrive channel 2,which .ipresents a cylindrical passage 22.along which .thevpercussionpin IZslidesand a pair .of diametrically. opposed grooves l23,rof rectangular section, :alongwhich the .jawsf20slide.

As shown. indFIGS. l2. and 8, these groovesJZB haveua constant section in their upper portion and then diverge substantially in the form of a pyramid as they approach the firing mouth 24, from which they are separated by a pair of diametrically opposed stops 25.

The drive channel 2 is in effect formed by joining together two almost equal plates 2a, 2b (FIGS. 3 7) which comprise rounded groves 22a, 22b respectively, which when together form the cylindrical passage 22, square grooves 23a, 23b respectively, which when together form the grooves 23, projections 25a, 25b repsectively, which when together form the stops 25 and semispherical external cavities 26a, 2612 respectively which allow the firing head of the gun to rest on the head of an already driven tack in order to drive tacks very closely together. The plate 2b also comprises a T opening 27 which communicates with the outlet of the magazine 3 so as to allow tacks to be introduced into the firing channel.

With the structure heretofore described, when a tack 28 is fed from the magazine 3 to the firing channel through the T opening 27, its head 29 goes to rest on the lower ends 21 of the jaws 20, which are maintained in the closed position by the opposing action between the middle portions and the non-diverging upper portions ofthe grooves 23, and is thus maintained in a suspended position with the shank 30 facing downwards (FIG. 2).

' In order to fire the tack into a layer 31 of material to be riveted, it is sufficient to push the trigger 6. On doing this, compressed air is introduced into the upper chamber of the cylinder 8, which causes rapid movement of the piston 11 and consequent advancement of the percussion pin 12 in the firing channel 2 rigid with the jaws 20 and the tack supported by them. As the upper portion of the grooves 23 is of constant section, the jaws 20 are initially maintained in closed position in spite of the opening thrust which the spring 17 exerts on the upper inclined ends of said jaws, because of which during the first part of its journey the tack 28 is supported and guided so as to avoid any undesirable separation, inclination or deformation of its head 29 and at the same time ensure its advancement perfectly perpendicular to the layer 31 of material to be riveted. When the jaws 20 arrive at the lower diverging portion of the grooves 23, the opening thrust of the spring 17 is no longer opposed and the jaws 20 open, freeing the tack 28.which proceeds on its journey thrust only by the percussion pin 12, whereas the jaws 20 stop against the stops 25 (their upper ends 19 slide in the openings 18 of the percussion pin, compressing the spring 17, which thus acts as a shock absorber). As shown in FIG. 8, this occurs only close to the end of the advancing journey of the tack and in particular when the shank of the tack has already been partially driven into the material 31, i.e., when it is no longer necessary to exert a guide action on the tack.

After the advancement stroke of the percussion pin 12 and hence when the tack has already been completely driven into the material 31, the piston 11 is returned into the starting position of FIG. 2 and consequently the percussion pin 12 rises and the jaws 20, reentering the portion of thegrooves 23 of constant section, close again and arrange themselves for supporting a new tack fed from the magazine 3.

It should be noted that the magazine 3 has not been described in detail and will not be hereinafter. This is due to the fact that it forms the object of my application Ser. No. 209,742, filed Dec. 20, I971, to which reference is made for its more detailed description. With regard to the description of the gun according to the present invention it is sufficient to know that it comprises essentially a feed channel 32 communicating with the two openings 27 of the firing channel, in which the tacks 28 are disposed in a row with their heads one against the other, and a pusher 33 which a spring 34 pulls against the row of tacks so as to thrust them towards the opening 27.

The gun shown in FIGS. 9 11 is analogous to that of FIGS. 1-8 with the exception that the upper ends 19 of the jaws 20 (similar parts of the two embodiments are indicated by the same reference numerals) are bent at 90 instead of with respect to the middle portions of the respective jaws, and that said upper ends 19 comprise teeth 35 facing outwards which slide in grooves 36 formed in the bottom of the grooves 23, keeping the jaws 20 closed until they meet stops 37 which cause the jaws 20 to partially tilt and consequently open in the lower diverging portion of the grooves 23 (FIG. 10). The grooves 36 are formed by joining together the grooves 36a, 36b respectively of the two plates 2a, 2b, and the stops 37 are formed by joining together the stops 37a, 37b, respectively of said plates.

In the embodiment shown in FIGS. 12 and 13 stops 25 perform the same function in relation to the lower ends 21 of the jaws 20 in the embodiment as do the stops 37 shown in FIGS. 9 to 11 (the lower diverging portions of the grooves 23 prevent excessive opening of the jaws, which could otherwise cause them to leave the drive channel through the openings between the lower ends of the grooves 23 and the stops 25), which otherwise is entirely analogous in construction to the constructions of FIGS. 1 8 and 9 11 and consequently carry the same reference numerals.

Some differences exist in the embodiment shown in FIGS. 14 15, in which inside the working head 51 there is a lower chamber 54 and an upper chamber 61 of each which can be connected alternately (by known means) to the supply of compressed air and to discharge. Inside the two chambers 54 and 61, separated by a gasket ring 63, there is a cylinder 58 inserted whose interior communicates with the chamber 54 v through holes 64, and 66, and with the chamber 61 through a hole 67. Inside the cylinder 58, which is closed at its lower end by a pad 60 and opened and closed at its upper end by a valve 59, two pistons 82 and 83 are slidably housed one sliding within the other (sealed by a gasket ring 85) so as to define an intermediate chamber 84 which communicates through a hole 86 with an annular chamber 87 defined between the external piston 82, the wall of the cylinder 58 and two gasket rings 88 and 89.

A cylindrical tube 62 is rigid with the piston 83, and to its lower end there are hinged the upper ends 69 (bent at right angles inwards) of two jaws 70 with their lower ends 71 bent at right angles inwards and their middle portions comprising ridges 68 projecting inwards. The two jaws 70 are housed in the internal passage of a firing channel 52 entirely similar to the drive channel 2 of the embodiment shown in FIGS. 1 8 and consequently provided with a cylindrical passage 72,

two grooves 73 of constant upper section and diverging toward their lower ends, and two stops 75 which separate the grooves 73 from the firing mouth 74. In this case the drive channel is also formed by joining together two almost equal plates, one of which comprises a T opening 77, through which a magazine 53 feeds tacks 78 composed of a head 79 and a shank 80.

A rod 55 is fixed to the piston 82 by a nut 90 and is provided with'two enlargements for sealing and guiding, 91 and 57, and an enlarged end head 56 which constitutes the percussion pin of the gun. The rod 55 is entirely housed within the piston 83 and the rod 62, with the exception of the enlarged end head 56 which remains below the lower end of the rod 62 so as to act as a finder for this latter.

In operation, the startingcondition is that shown in FIG. 14, with the chamber 61 connected to the compressed air supply, the chamber 54 connected to discharge in known fashion (this forms no part of the present invention) and the valve 59 in the closed position. Compressed air then enters the chamber 84 through the communicating holes 67 and 86 and holds the pistons 82 and 93 apart at the upper end of the cylinder 58, consequently maintaining the percussion pin 56 in the raised position, and the jaws 70 inserted in the upper portion of constant section of the grooves 73 so that they are closed in order to support the head of a tack 78 fed from the magazine 53 through the opening 77.

in order to fire the tack into the material 81, it is sufficient to raise the valve 59. The compressed air then thrusts thepiston 82 downwardsand consequently also the. piston 83 (which is kept apart by the compressed air iinprisoned in the chamber 84), because of which the rod 62 with thejaws 70 and the rod 55 with the percussion pin 56' advance in the firing channel towards thefiring mouth 74. Movement of the two pistons proceeds in this manner until the hole86 of the piston 82 reaches a position registering with the hole 66 of the cylinder 58. At that point the compressed air imprisoned in the chamber 84 flows to discharge, and whereas the piston 83 stops against the pad 60, the piston 82proceeds on its stroke until it comes into contact with the piston 83. At this, the rod 55 slides in the rod 62 because of which the percussion pin 56 acts on the ridges 68 of the jaws 70, so causing them to open. The opening of the jaws 70, permitted because said jaws reach the lower diverging portion of the grooves 73, liberates the head of the tack 78, which thus proceeds on its driving journey under the thrust only of the percussion pin 56 (FIG. In this case, by suitably arranging the hole 66 and the diverging portions of the grooves 73, it is also possible for the tacks to be freed only when the shank 80 has already been partially driven into the material 81.

For the return, when the compressed air has been discharged from the cylinder 58, compressed air again enters the chamber 84 from the chamber 54 through the holes 65 and 86 so as to again push the pistons 82 and 83 back, then the compressed air entering through the holes 64 raises the two pistons rigidly so that they return to the raised position shown in FIG. 14. Supply of compressed air alternately to the opposite ends of the pistons, and exhaust of the air from opposite ends of the pistons, respectively, can be controlled by any conventional valve means, manually operated, for instance, and forming no part of the present invention.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A gun for driving tacks or wide-headed nails into a material, comprising a member formed with a drive channel into which is fed the articles to be driven, and a percussion pin which moves in said member in order to thrust the articles out through the firing mouth of the gun and into the material, and having gripping means supported by the percussion pin and inserted in the drive channel and movable with said pin through the drive channel,

said drive channel comprising a first portion having a constant width and a second portion with its width increasing progressively toward said firing mouth, whereby during the advance stroke of the percussion pin toward the firing mouth while said gripping means is moving in said first portion of said drive channel, said gripping means is maintained in closed position supporting the head of a tack until said percussion pin approaches the end of its advance stroke, and

resilient means constantly urging said gripping means toward open position, whereby, when said gripping means is moving in said second portion of said drive channel, said gripping means is automatically moved to open position in order to free the head of the tack when the shank of the tack is already partially inserted in the material to allow said tack to proceed freely, under continued thrust of said percussion pin, onto insertion into the material under continued thrust of said percussion pin.

2. Agun as claimed in claim 1, in which saidpercussion pin comprises a shaft within which is housed a rod with enlarged end heads, and about which is wound a spring acting between one of said end heads and a striker which slides on said rod, and in which said gripping means comprise at least two jaws having their ends which face the firing mouth bent inwards so as to constitute resting members for the head of the tack, and the opposing ends obliquely bent inwards and inserted between said sliding striker and the other end head of said rod through longitudinally elongated openings in the shaft of the percussionpin, and in which said drive channel is provided with an internal passage formed from a central bore of constant section in which the percussion pin slides and at least two longitudinal channels in which the jaws slide, each of said channels having a portion of constant section distant from the firing mouth and a portion close to the firing mouth which diverges as it approachessaid firing mouth, from which it is separated by an end stop for the respective jaw, said portions of constant section constituting the first portion of said channel member, and said diverging portions constituting said second section of said channel member.

3. A gun as claimed in claim 1, which comprises stop means inserted in the path of advance of the gripping means so as to cause them to move into an open position on their arrival at the second portion of the firing channel.

4. A gun as claimed in claim 3, :in which said percussion pin comprises a shaft within which is housed a rod with enlarged end heads, about which is wound a spring acting between one of said end heads and a striker which slides on said rod, and in which said gripping means comprise at least two jaws with their ends facing the firing mouth bentinwards so as to constitute resting members for the head of the tack, the opposing ends being bent inwards and inserted between said sliding striker and the other end head of said rod through longitudinally elongated openings in the shaft of the percussion pin, and said jaws having teeth projecting outwards close to said opposing ends, and in which said drive channel is provided with an internal passage formed from a central bore of constant area in which the percussion pin slides, at least two longitudinal channels in which the jaws slide, each of said channels having a first portion of constant section distant from the firing mouth and a second portion close to the firing mouth which diverges as it approaches the firing mouth from which it is separated by an end stop for the respective jaw, and two longitudinal grooves in which said teeth projecting from the jaws slide, said grooves extending together with said portions of the channels and terminating in end stops situated at the beginning of said second portions of the channels, said first portions of constant section together constituting the first portion of said channel member, and said diverging portions constituting said second section of said channel member.

5. A gun as claimed in claim 3, in which said percussion pin comprises a shaft within which is housed a rod with enlarged end heads, about which is wound a spring acting between one of said end heads and a striker which slides on said rod, and in which said gripping means comprise at least two jaws having their ends adjacent the firing mouth bent inwards so as to constitute resting members for the head of the tack, the opposite ends of said jaws being bent inwards and inserted between said sliding striker and the other end head of said rod through longitudinally elongated openings in the percussion pin shaft, and in which said drive channel is provided with an internal passage formed from a central bore of constant section in which the percussion pin slides and at least two longitudinal channels in which the jaws slide, each of said longitudinal channels having a first portion of constant section distant from the firing mouth and a second portion close to the firing mouth which diverges as it approaches the firing mouth, from which it is separated by an end stop with a striking face inclined downwardly outwardly, said first portions of constant section together constituting the first portion of said channel member, and said diverging portions constituting said second section of said channel member.

6. A gun as claimed in claim 3 in which said gripping means comprise at least two jaws having their ends adjacent the firing mouth bent inwards so as to constitute resting members for the head of the tack, middle portions comprising at least one ridge projecting inwards, and their opposite ends hinged to a shaft in which a rod is slidingly housed provided with an enlarged end head which constitutes the percussion pin, and in which the drive channel is provided with an internal passage formed from a central bore of constant section in which the shaft and percussion pin slide axially and at least two longitudinal channels in which the jaws slide, each of said channels having a first portion of constant section distant from the firing mouth and a second portion close to the firing mouth which diverges on apporaching said firing mouth, and in which means is provided for controlling said shaft and said rod so as to move rigidly together for a first part of their movement of advance towards the firing mouth until the jaws arrive at the second diverging portions of the drive channel, after which the shaft stops whereas the rod continues its movement, allowing the percussion pin to wedge itself between the projecting ridges of the jaws so as to cause said jaws to open.

UNITED STATES PATENT OFFICE "CERTIFICATE OF CORRECTION Patent No. 3, 764,051; Dated October 9,' 1973 Invent Umberto Monacelli I v It iseerti'fied that errorappears in the above-identified patent I and that said Letters Patent are hereby corrected as shown below:

In column I of the front page after "451.7 Appl. NO. 2109,6 46" insert -Foreign Application Priority Data December 22, 1970 Itaiy- -+33443A/7o--.

Signed and sea led this 9th day ofJuly 1 974.

(SEAL) rAttest:

oY M. GIBSON JR. t c. MARStIALL DANN c esting Officer Commissl on'er of Patents romeo-1050 (10-69) 

1. A gun for driving tacks or wide-headed nails into a material, comprising a member formed with a drive channel into which is fed the articles to be driven, and a percussion pin which moves in said member in order to thrust the articles out through the firing mouth of the gun and into the material, and having gripping means supported by the percussion pin and inserted in the drive channel and movable with said pin through the drive channel, said drive channel comprising a first portion having a constant width and a second portion with its width increasing progressively toward said firing mouth, whereby during the advance stroke of the percussion pin toward the firing mouth while said gripping means is moving in said first portion of said drive channel, said gripping means is maintained in closed position supporting the head of a tack until said percussion pin approaches the end of its advance stroke, and resilient means constantly urging said gripping means toward open position, whereby, when said gripping means is moving in said second portion of said drive channel, said gripping means is automatically moved to open position in order to free the head of the tack when the shank of the tack is already partially inserted in the material to allow said tack to proceed freely, under continued thrust of said percussion pin, onto insertion into the material under continued thrust of said percussion pin.
 2. A gun as claimed in claim 1, in which said percussion pin comprises a shaft within which is housed a rod with enlarged end heads, and about which is wound a spring acting between one of said end heads and a striker which slides on said rod, and in which said gripping means comprise at least two jaws having their ends which face the firing mouth bent inwardS so as to constitute resting members for the head of the tack, and the opposing ends obliquely bent inwards and inserted between said sliding striker and the other end head of said rod through longitudinally elongated openings in the shaft of the percussion pin, and in which said drive channel is provided with an internal passage formed from a central bore of constant section in which the percussion pin slides and at least two longitudinal channels in which the jaws slide, each of said channels having a portion of constant section distant from the firing mouth and a portion close to the firing mouth which diverges as it approaches said firing mouth, from which it is separated by an end stop for the respective jaw, said portions of constant section constituting the first portion of said channel member, and said diverging portions constituting said second section of said channel member.
 3. A gun as claimed in claim 1, which comprises stop means inserted in the path of advance of the gripping means so as to cause them to move into an open position on their arrival at the second portion of the firing channel.
 4. A gun as claimed in claim 3, in which said percussion pin comprises a shaft within which is housed a rod with enlarged end heads, about which is wound a spring acting between one of said end heads and a striker which slides on said rod, and in which said gripping means comprise at least two jaws with their ends facing the firing mouth bent inwards so as to constitute resting members for the head of the tack, the opposing ends being bent inwards and inserted between said sliding striker and the other end head of said rod through longitudinally elongated openings in the shaft of the percussion pin, and said jaws having teeth projecting outwards close to said opposing ends, and in which said drive channel is provided with an internal passage formed from a central bore of constant area in which the percussion pin slides, at least two longitudinal channels in which the jaws slide, each of said channels having a first portion of constant section distant from the firing mouth and a second portion close to the firing mouth which diverges as it approaches the firing mouth from which it is separated by an end stop for the respective jaw, and two longitudinal grooves in which said teeth projecting from the jaws slide, said grooves extending together with said portions of the channels and terminating in end stops situated at the beginning of said second portions of the channels, said first portions of constant section together constituting the first portion of said channel member, and said diverging portions constituting said second section of said channel member.
 5. A gun as claimed in claim 3, in which said percussion pin comprises a shaft within which is housed a rod with enlarged end heads, about which is wound a spring acting between one of said end heads and a striker which slides on said rod, and in which said gripping means comprise at least two jaws having their ends adjacent the firing mouth bent inwards so as to constitute resting members for the head of the tack, the opposite ends of said jaws being bent inwards and inserted between said sliding striker and the other end head of said rod through longitudinally elongated openings in the percussion pin shaft, and in which said drive channel is provided with an internal passage formed from a central bore of constant section in which the percussion pin slides and at least two longitudinal channels in which the jaws slide, each of said longitudinal channels having a first portion of constant section distant from the firing mouth and a second portion close to the firing mouth which diverges as it approaches the firing mouth, from which it is separated by an end stop with a striking face inclined downwardly outwardly, said first portions of constant section together constituting the first portion of said channel member, and said diverging portions constituting said second section of said channel member.
 6. A gun aS claimed in claim 3, in which said gripping means comprise at least two jaws having their ends adjacent the firing mouth bent inwards so as to constitute resting members for the head of the tack, middle portions comprising at least one ridge projecting inwards, and their opposite ends hinged to a shaft in which a rod is slidingly housed provided with an enlarged end head which constitutes the percussion pin, and in which the drive channel is provided with an internal passage formed from a central bore of constant section in which the shaft and percussion pin slide axially and at least two longitudinal channels in which the jaws slide, each of said channels having a first portion of constant section distant from the firing mouth and a second portion close to the firing mouth which diverges on approaching said firing mouth, and in which means is provided for controlling said shaft and said rod so as to move rigidly together for a first part of their movement of advance towards the firing mouth until the jaws arrive at the second diverging portions of the drive channel, after which the shaft stops whereas the rod continues its movement, allowing the percussion pin to wedge itself between the projecting ridges of the jaws so as to cause said jaws to open. 